The present invention relates generally to cellular mobile telephone systems, and more specifically to a channel selection technique for selecting a control channel during call setup periods.
According to the EIA or TACS cellular mobile telephone system, idle mobile stations are normally tuned to an assigned standby channel to receive identifiers of call setup channels specified by the standby channel. When originating or receiving a call, the mobile station detects the signal levels of the specified call setup channels and selects one having the highest signal level and transmits a call-request signal or returns an answer signal over the selected channel.
Recent demands for mobile telephone systems have given rise to the establishment of similar mobile telephone systems by different common carriers, using a frequency reuse scheme. However, in some fringe areas, an undesirable situation can occur in which a certain frequency band is shared between adjacent cells served by different common carriers. The mobile station is constantly receiving call setup channel identifiers specified by the strongest standby channel of its own system. If the up-direction call setup channel that is paired with the strongest down-direction call setup channel is shared with the subscribers of a different common carrier in an adjacent cell, the signal levels of the different common carrier may, in some cases, be stronger than those of its own signal. If a call is attempted under such circumstances, it is likely that the mobile station will seize a call setup channel of the different common carrier.